Method of making a resinimpregnated capacitor



J. BURNHAM July 20, 1954 METHOD OF MAKING A RESIN-IMPREGNATED CAPACITORFiled Oct. 9. 1950 INVENTOR. (/0/9/V BURN/754M M v ///5 flrrafi/vfkPatented July 20, 1954 METHOD OF MAKING A RESIN- IMPREGNATED QAPACITORJohn Burnham, Williamstown, Mass, assignor to Sprague Electric Company,

North Adams,

Mass, a corporation of Massachusetts Application October 9. 1950, SerialNo. 189,250

3 Claims. 1

This invention relates to capacitors that are impregnated with resinwhich is solidified in place to produce a thermosetting mass.

Cross-linked and thermosetting resins have been previously incorporatedin condensers but have heretofore been not too satisfactory because ofthe frequent presence of cracks and voids in the completed condenserbody, and also because the external surface of the curedresinimpregnated assembly has always been irregular, the resin on thesurface appearing as if it has been preferentially flowed to one portionor end, producing a relatively thick unsightly glob or drop. The curingof the impregnated resin during the hardening step has usually resultedin the development of voids within the condenser in spite of allprecautions. In addition the impregnant before and during curing dripsfrom the capacitor to spoil its appearance, and this cannot be preventedby wiping the surface of the impregnated capacitor prior to heat curmg.

Among the objects of the present invention are novel techniques foravoiding the above difficulties, as well as to produce improvedelectrical capacitors.

The above as well as further objects of the invention will be morereadily understood from the following description of several of itsexemplifications, reference being made to the accompanying drawingwherein the figure is a sectional view of one form of condenser made inaccordance with the present invention.

It has been discovered that polymerizable impregnants can be used tomake void-free and drop-free impregnated capacitors if the con densersimpregnated with monomer or low polymer are immersed in an inert liquidsolvent for the impregnant, and the liquid is maintained at an elevatedtemperature until the resin be comes gelled and the peak of the exothermis passed. The capacitor can then be removed from the liquid, dried, andbaked in air till the curing is completed and the resin is fully solidi-The liquid solvent acts as a heat transfer medium to control theexothermic polymerization reaction and to remove excess solvent from thesurface of the impregnated capacitor.

By way of example only, the following process has been found to produceexcellent results. Two ribbons of aluminum foil 0.00025" thick, one inchwide and '72 inches long were convolutely wound in the conventionalmanner with two ribbons of calendered kraft paper 0.0005" thick and oneinch wide. The foils were displaced transversely with respect to eachother and with respect to the paper so that their opposite side marginsextended out from the opposite ends of the wound assembly to provide thestandard non-inductive capacitor section. Leads were then soldered tothe respective side margins with aluminum solder and the entire sectioninserted in a cardboard tube. The ends of the tube were filled with acement made up from 23 parts of melamine formaldehyde resin, 63 parts ofground silica, and 14 parts of water. The assembly was placed in acontainer which was evacuated to a pressure of 500 microns of mercury.While at this low pressure, the capacitor assembly was immersed in aliquid resin mixture (25 C.) having the following formulation:

Parts by weight Ingredient:

Propylene glycol fumarate (made by heating together stoichiometricproportions of fumaric acid and propylene glycol at 180 C. for 3 hours)Styrene 70 Tertiary butyl hydroperoxide 1 The pressure was then broughtup to atmospheric and after a few minutes the sections were removed fromthe impregnant, and immersed in methyl isobutyl ketone held at loo-119C. After thirty minutes of immersion in this liquid, the impregnantthroughout the section was in the form of a semi-hard. gel, and theassembly removed, dried and baked at 150 C. for three hours in a hot airoven.

The final assembly was hard and appeared to be fully cured. It was givena wax dip and was ready for use. It had a D. C. breakdown potential inexcess of 3500 volts D. 0., high leakage resistance and a life of over500 hours at C. under an applied potential of 600 volts D. C. Twentycapacitors made in this way were sawed in two and no voids could bedetected within the capacitor section in any case. Their externalsurfaces had a uniform drop-free thin film of solid resin below the waxcoating.

The figure of the drawing shows the final condenser. The foils areindicated at m and H, the paper spacers at l2 and t3, the terminals atE4 and i5, and the soldered connections at I5 and [7. End cement 2! and22 fill the ends of the capacitor, mechanically fixing the assembly withcardboard tube 23. The solidified impregnant i3 fills the spaces andpores in the entire capacitor and also extends as a thin film 3 4 :5QVQY th exterior. A Wax coating 20 covers pregnants at normaltemperatures, but under this resin film. the conditions of gelation, thesolvent effect In additign t th above d t il d l should be sufiicient toremove the X8858 imnumerous other dielectric impregnants of the g antWhich is found On theouter Surface pogymerizable type may be processedin accorr 5 or the capacitor sect on. It should he recognized ance withthe invention As previously indi that the porosity or the capacitorstructure is an important factor in the selection on a solvent. Forexample, the medium employed in an uncased capacitor section should notpenetrate the capacitor Winding appreciably due to its solvent action.Where a molded casing is employed,

heat generated by the lmpregnant better solvents may be safely employed,as for re polymerization reaction. Representative example, Xylene fstyrene impregnants,

ceted, the medium should be inert to the polymerization reaction, asolvent for the impregnant under conditions of gelation, and liquidduring the gelation reaction, so as to remove the exgnants, processingmediums and gelatio In addition to the solute-solvent interactionstemperatures are indicated in the table which which have been discussedabove t 13 follows. and heat conductivity of the solvent is of im-Impregnsnt Liquid Medium g and N-vinyl curbazolc-l-O.5% benzoylperp-xylcnc 1 hour at 115 C.

oxide.

slyicilt; 12% p-divinyl benzene. di-n-butyl ketone 45 minutes at 100 C.1 -11 a ilorostyrene propylene glycol... 2 hours at 125 C.

glycol funicmte; 9% cthylcncglycol- 90 min. at 110 C.

. icrtbutyl li \'dropcrmono-methyl idc. other acetate.

vinyl dillicnzoiurrm plus 0.3% acetyl triacetln 40 min. at 135 Similarresults may be obtained when alcohols, portance and should be chosen togive the ethers, esters, aldehydes, ketones and other inert mum heattransfer from the component to liquids, including hydrocarbons, are usedas the bull: of the liquid. immersion control mediums, either by them-The exothermic polymerization reaction leads selves or mixtures,selected in accordance with to appreciable heat formation. The rateinvention. transfer of this heat to the liquid depends ui Examples ofthese hqulds the initial ambient polyme 1 etion temp i the structuralconfiguration of the capacitor, etc.,

Ethyl isobutyl ketone as Well as the nature of the solvent. The gela-Di-isobutyl ketone tion point referred to herein means the poi." n-butylketone at Which impregnant dripping will no lo 353511346116 3mono-methyl Ethel take place and at which the peak of the exotl 1 t y 5mono-methyl Ethel acetate 40 is passed. Extremes may he i let in thecase of n-Amyl alcohol. impregnants containing substantial quantities t73 n-butyl ether active polymerizants such as 2,5-dichloro- Ethylbutyrate styrene, divinyl benzene and the like. To Tetralin prove theheat transfer action, agitation of the Methyl henzyl ether 1 liquidmedium may be necessary. Methyl benzoate The housing for the capacitoror other clectri- B 112 1 alcohol cal device may be a cardboard tube,usual] v 1 Methyl hetachloro propionate a semi-porous end cement, whichin tu- Benzaldehyde be provided with an eyelet to facilitate imprerlitrobenzene nation; a molded thermosetting resin cash Methyl m-chlorobenzoate metal can with terminal eyelets; a o Brethyl carbonate solubleresin tube with metal end caps or ct. Ani -ole type of housingconventionally applied for such Durene devices. Methyl lactate It shouldbe further understood that the inven- Cyclohexanol tion is not limitedto electrical capacitors; trans- Cyclchexanone formers, coils and evenresistors imprec Propylene glycol with the impregnants described hereinarray he Acetal processed in accordance with the invention. Dioxane G0The resins employed herein are polyineri role Beta chloroethyl acetatecompounds containing one or more u" aturated O-terphenl carbon doublebonds. The materiis may be With some liquids such as n-propyl alcoholand employed per Se as tt with other polymethyl ethyl ketonesuperatmospheric pressures menzable mat.ena1s' 9 can be used to raisetheir boiling points and pro 5 of such mater la15 a ff vide an effectiveresin gelling treatment. Ordihowever the mvent,10n oYercpmes i necebswynaiily temperatures between about 80 C. and Separate pl'e"p?lymel'lz,atlons 0 are employed to effect gelation since theeaothermicreaction encountered dur- The selection of the liquid medium depends ingthe initial phases effects no disadvantage of course, upon theparticular polymerizable mawhen the Present invention is Q Q F 3tel-191s employed as impregnants It is normany merizable vinylcompounds, based on the aropreferable to employ higher boiling materialsmatic, aliphatic and heterocyclic nuclei and unwith the resins whichrequire higher temperasaturated polyesters r representative r in curesfor gelation. Many of the liquid mediums materials. are not particularlyeffective solvents for the im- Details of the ester production aredescribed in the Ellis Patent No. 2,255,313 granted September 8, 1941,which also shows polymerization mixtures, catalysts and stabilizers thatcan be used. In addition to these catalysts and the tertiary butylhydroperoxide described above, acetyl peroxide, benzoyl peroxide andlauryl peroxide have been found effective with the present invention.

The impregnation can be performed at pressures higher or lower than 500microns of mer cury. The impregnation can be repeated, particularly ifthe pressures are cycled.

As many apparently widely different embodimerits of this invention maybe made without departing from the spirit and scope hereof, it is to beunderstood that the invention is not limited to the specific embodimentshereof, except as defined in the appended claims.

What is claimed is:

1. In a method of making a capacitor, the

steps of providing a capacitor section having electrodes separated byporous spacer elements, impregnating the capacitor section with at leastone polymerizable material, immersing the impregnated section in aninert liquid medium which is a solvent for said polymerizable material,said immersing being effected at a temperature above C. until theimpregnant within the section has become gelled, removing from saidsolvent and then heating the resulting section till the impregnant iscompletely cured.

2. A method of making a capacitor including the steps of rolling up anarmature assembly with porous spacing material; impregnating the rolledassembly with a liquid mixture of styrene and linear polyester of anacid selected from the class consisting of maleic and fumaric acids;washing the impregnated assembly with a ketone heated to about C. toremove excess im pregnant from the surface of the assembly and effectgelation of the impregnant; and then withdrawing the assembly from theketone and heat-- ing the Withdrawn assembly to complete the curing ofthe impregnant.

3. A process of making a capacitor comprising the steps of rolling up anarmature assembly with porous spacing material, impregnating the rolledassembly with a liquid mixture of styrene and propylene glycol fumarate,and a peroxide poly merization catalyst, immersing the impregnatedassembly for a period of about 30 minutes in methyl isobutyl ketone atfrom about 100 to about C. so as to form a semi-hard gel of theimpregnant throughout said assembly, withdrawing the assembly fromketone and heating the Withdrawn assembly at about C. for about 3 hoursto complete the curing of the impregnant.

lteferences Cited in the file of this patent UNITED STATES PATENTSNumber Name Date 2,350,743 Fordyoe June 6, 1944 2,479,357 Hill Aug. 16,1949 2,520,173 Sanders Aug. 20, 1950 2,549,309 Hill Apr. 17, 1951FOREIGN PATENTS Number Country Date 416,735 Great Britain Sept. 20, 1934596,151 Great Britain Dec. 30, 1947 597,218 Great Britain Jan. 21, 1948

1. IN A METHOD OF MAKING A CAPACITOR, THE STEPS OF PROVIDING A CAPACITORSECTION HAVING ELECTRODES SEPARATED BY POROUS SPACER ELEMENTS ,IMPREGNATING THE CAPACITOR SECTION WITH AT LEAST ONE POLYMERIZABLEMATERIAL, IMMERSING THE IMPREGNATED SECTION IN AN INERT LIQUID MEDIUMWHICH IS A SOLVENT FOR SAID POLYMERIZABLE MATERIAL, SAID IMMERSING BEINGEFFECTED AT A TEMPERATURE ABOVE 80* C. UNTIL THE IMPREGNANT WITHIN THESECTION HAS BECOME GELLED, REMOVING FROM SAID SOLVENT AND THEN HEATINGTHE RESULTING SECTION TILL THE IMPREGNANT IS COMPLETELY CURED.